Researches and proposals have been made for adding secondary components to silicon nitride for improving sinterability and machining properties of silicon nitride. For example, a method for adding MgO and ZrO.sub.2 at specific ratios has been applied for patent in the name of the present Applicant and published under the Japanese Patent Kokoku Publications Nos. 60-16388(1965) and 60-20346(1985). Sintered bodies containing strontium (Sr), magnesium (Mg), cerium (Ce) and zirconium (Zr) compounds or additionally containing aluminum compounds have also been proposed under the Japanese Patent Kokoku Publication No. 61-40621(1986) and Japanese Patent Kokai Publication No. 62-65976(1987).